270 



THE EVOLUTION THEORY 



male. In the former the cells remain as they were before, but in the 

 male colonies the sixteen or thirty-two cells undergo a peculiar 

 process of division, which ends in each becoming a mass (16-32) of 

 so-called ' zoosperms,' that is, minute, narrow, longitudinally elongated 

 cells with two flagella (Fig. 63 at D shows those of Volvox). In 

 Eudorina they differ from the female germ-cells or ova externally in 



Fig. 63. Volvox aureus, after Klein and Schenok. 

 A, besides the small flagellate somatic cells of the colony- 

 there are five large egg-cells (i) which are capable of 

 parthenogenetic development, three recently fertilized egg- 

 cells (0) and a number of male germ-cells (a) in process of 

 multiplication. From each of these, by continued division, 

 a bundle of spermatozoa arises. B, a bundle of thirty-two 

 sperm-cells in process of development, seen from above. 

 0, the same seen from the side. Magnified 687 times. 

 D, individual spermatozoa, magnified 824 times. 



form and size, as well as by being much more actively motile, and 

 they contain green and subsequently yellow colouring matter, and 

 a red eye-spot. We here find, for the first time among multicellular 

 organisms, the dififerentiation of male and female germ-cells ; and we 

 learn from this that the essence of fertilization does not lie in this 



